1. Field of the Invention
The present invention relates to a ceramic oxide superconductive composite material which comprises a ceramic oxide superconductor and a non-superconductive material comprising at least one element which does not react with any of the elements of the ceramic oxide superconductor. More particularly, it relates to a ceramic oxide superconductive composite material having a new construction whereby the ceramic oxide superconductor having a higher superconductive critical temperature can be effectively utilized.
2. Description of Related Art
A superconductor exhibits complete diamagnetism under superconductive conditions and has no potential difference therein in spite of flow of stationary current in therein. By using this characteristic of the superconductor, many applications of superconductors as mediums for electric current have been proposed.
Application fields of superconductor include versatile technical fields such as an electric power field (e.g. MHD power generation, power transmission, energy storage, etc.), a motive power field (e.g. linear motor (magnetic levitation) trains, electromagnetic propulsion ships, etc.), and a measuring field in which the superconductor is used as a highly sensitive sensor for a magnetic field, a microwave, radiation and the like (e.g. NMR, therapy using .eta.-meson, high energy physical experiment apparatus, etc.).
In addition, in the electronics including a Josephson element, the superconductor is expected to provide an element which can not only decrease power consumption but also work at a very high speed.
The superconductive phenomenon has been found in specific metals or organic compounds at extremely low temperatures. Namely, among classical superconductors, Nb.sub.3 Ge is said to have the highest critical temperature Tc for superconductivity of 23.2K and this temperature has been considered as the upper limit of Tc for a long time.
Hitherto, to realize the superconductive phenomenon, a superconductor should be cooled to a temperature lower than Tc with liquid helium which has a boiling point of 4.2K. However, the use of liquid helium greatly increases technical burden and cost because of necessity of a cooling system including an apparatus for liquefying helium, which prevents practical application of the superconductor.
Recently, it was reported that a sintered material comprising oxides of elements of IIa or IIIa group of the periodic table can act as a superconductor at a high critical temperature and is expected to accelerate practical application of superconductor technology. From already available reports, compound oxides having a crystal structure similar to the perovskite crystal structure such as (La,Ba).sub.2 CuO.sub.4 or (La,Sr).sub.2 CuO.sub.4 are known as superconductors having high Tc. These compound oxides have Ic of 30 to 50 K, which is far higher than that of the classical superconductors. In addition, a Ba-Y-Cu type superconductor is reported to have Tc higher than the liquid nitrogen temperature although its structure has not been identified.
Since the ceramic oxide superconductor is prepared by sintering powdery metal compounds as raw materials, it is inevitably porous. Due to porosity, the ceramic oxide superconductor has various drawbacks such that (a) an amount of electric current which can flow through the superconductor is small, namely a current density is small, (b) the superconductor has low mechanical strength and easily broken, and (c) if water is trapped in pores, hydroxide groups of water react with the material to deteriorate the superconductor.
It is known that the presence of oxygen in a sintering atmosphere greatly influences properties of superconductor in the preparation of ceramic oxide superconductor. For example, the raw materials are sintered in the air, a surface part of sintered material, for example, a surface layer of thickness of 0.5 mm from the surface which readily contacts the air has superior superconductive characteristics, namely higher critical temperature than inner parts of sintered material, although non-surface parts of sintered material may have superconductive characteristics.
In view of strength, the ceramic oxide superconductor which is prepared as a sintered material is usually brittle and requires great care to handle. That is, the ceramic oxide superconductor is easily broken or cracked by mechanical stress. Particularly, the ceramic oxide superconductor in the form of a wire is very easily broken. When the raw materials are formed in a porous state or a three-dimensional network to increase a surface area so as to make use of oxygen supply on the surface during sintering and to achieve desired superconductive characteristics, the sintered product becomes more fragile and its mechanical strength further decreases. Accordingly, it is practically difficulty not only to plastically work the ceramic oxide superconductor but also to simply form. Therefore, its practical use has severe limitation.
Furthermore, it is very difficult to form a sintered superconductor from homogeneous polycrystal consisting of particles all having superconductive characteristics and, as a general property of the superconductor, the superconductive state may be locally broken by change of external magnetic field or cooling temperature. The ceramic oxide superconductor has smaller thermal conductivity and larger electrical resistance than the classical superconductors. Therefore, when the superconductive state is locally broken, such part of the superconductor is locally heated by electrical current which flows through the superconductor. If the cooling medium contacts the locally heated part of superconductor, it is explosively vaporized.
To prevent such explosive vaporization, the classical metal superconductor is processed in the form of a thin filament and a plural number of filaments are bundled by a good conductive material such as copper, which acts as a thermal conductor and a by-pass of electric current in case of breakage of superconductive state. However, it is difficult to process the ceramic oxide superconductor having higher Tc in the form of filament.